1. Field of the Invention
This invention relates to frost detectors for refrigerating apparatuses, and more particularly to frost detectors which detect the deposition of frost on the air coolers disposed in the refrigerator chambers and generate signals for starting the defrosting operation.
2. Description of the Prior Art
In the course of operation of an refrigerating apparatus, frost tends to deposit on the air cooler disposed in the refrigerator chamber which stores fresh foods, etc. The air cooler usually comprises the pipes or finned coils in which the refrigerant evaporates and deprives heat therefrom, and the air blower which blows air past the pipes or finned coils and circulates the cooled air in the refrigerator chamber. Alternatively, brine may be used as the medium flowing through the pipes of the air cooler to deprive heat therefrom, the brine being cooled in a heat exchanger disposed in the evaporator of a refrigerating device.
Thus, when frost deposits on the air cooler, the air cooling ability and effectivity of the cooler drops, necessitating occasional defrosting thereof. The defrosting is effected by energizing a heater, or by spraying water from a spray nozzle, or by blowing hot gas past the coils.
Thus, it is desirable to provide the air cooler with a frost detector which detects the deposition of the frost on the air cooler and generates a signal for energizing the defroster. A conventional method of detecting the frost deposition has been based on the fact that the electric current flowing through the electric motor of the air blower increases as the frost accumulates on the air cooler. Thus, a conventional frost detector comprised a current detector which detects the current I flowing through the air blower of the air cooler, and a comparator which compares the value of the detected current with a predetermined fixed value I.sub.1 and generates a defroster activating signal when the detected value exceeds the fixed predetermined value I.sub.1.
This type of frost detector, however, has not been wholely immune to errors:
(1) The air blower current I, or the electric current flowing through the motor of the air flower, depends not only on the amount F of deposited frost, but also on the temperature T of the air within the refrigerator chamber. Thus, the timing of the defroster activating signal depends on the temperature of the air T, as well as on the amount of frost F.
(2) The air blower current I also depends on the voltage V applied on the air flower motor. Thus, the variation or fluctuation in the voltage V of the electric power source for the air blower may also cause the misfunction of the frost detector.
(3) When the path of the air flow is blocked by a big rubbish, for example, the air blower current I is affected and an erroneous signal may be generated.
(4) Gradual deterioration in the performance of the bearings of the air blower causes an increase in the mechanical load thereof, which in its turn causes the air blower current I to increase.
The alternative method of detecting the accumulation of frost on the air cooler relied on the fact that, as the frost accumulates on the pipes or finned coils of the air cooler, the difference between the temperature T of the air in the refrigerating chamber and the temperature S of pipes or coils increases. Thus, an other type of conventional frost detector comprises temperature detectors detecting the temperatures T and S of the air in the refrigerating chamber and of the air cooler respectively, and generates a defroster activating signal when the difference (T-S) in the detected temperatures T and S exceeds a predetermined fixed value .alpha..
This type of conventional frost detector was not immune to errors, either:
(5) The opening of the door of the refrigerating chamber momentarily raises the temperature T of the air therein, and thus may trigger the detector to generate an erroneous defroster activating signal.
(6) The low pressure of the refrigerant within the pipes or coils during the starting of the refrigerating system may also cause the misfunction of the frost detector.
(7) The refrigerant gas leakage in the refrigerating system also causes the pressure to decrease, which is a possible factor in bringing about the misfunction of the detector, as above.